Porous metal and method of forming the same



Patented Aug. 23, 1932 um'reo s'i CHARLES F. SHERWOOD, OF HIGHLAND PARK,MICHIGAN, ASSIGNOR TO SHERWOOD PATENTS, LTD., A CORPORATION OFCALIFORNIA POROUS METAL ANT) METHOD OF FORMING THE SAME No Drawing.

The present invention relates to bearings, bushings,or other articlesand to the method of manufacturing the same.

More particularly it relates to bearings 01' other articles composed ofsintered metal.

that'it may be made in large as well as in 1 small sizes, theparticlesof the powdered metals being united by coalescence.

In carrying out the process, the powdered metal is formed under pressureinto an agglomerate or briquette including a mixture go of the powderedmetal and a material of a lubricating character which lubricant has axvolatilization point below the temperature at which sintering of thepowdered metal takes place. ,The effect of the volatilization .25 of thelubricant below the sintering temperature is that there will be noresidue or extraneous material between the particles of metal, but onthe contrary, at the time the sintering temperature is reached thelubricating so binding material will have been volatilized and drivenoff leaving the metal particles in contact metal to metal at thejuxtaposed or contacting portions of their surfaces, so that coherenceand coalescence will take place 85 throughout these surfaces at thesintering temperature.

Another object of the invention is the posi tive reduction of anyexistent metallic oxides, the prevention of oxidation, and theproduction of a metallic body improved in strength, ductility andmachineability.

Other objects will appear from the following specification.

The improved process is applicable to the manufacture of bearings,bushings, brushes, brake linings and other articles from variouspowdered metals and mixtures thereof.

The art of manufacturing articles from 5o powdered metals is extremelyold, originating Application filed. November 18, 1929. Serial 1T0.407,022.

ilrsifagllannheim, Germany, in approximately The old method of procedurewas, briefly, making use of finely divided metallic powders and byfinely divided, I refer to metal particles that will pass 150 meshscreendry, and under pressure forming them into a coherent briquette,placing the same, after formation, in heat for varying periods of t1me,and alloying the metal by the heat at a temperature lower than that ofthe melting point of't-he main constituent metal. For example, a mixtureof 84% copper, 10% tin, and 6% graphite, will alloy and sinter atatemperature of 1425 F. The melting point of copper is 1820 F. and thatof tin approximately 400 F so that it will be seen that the sinteringtemperature is a mean of the two temperatures of the constituent metals.This method has been in use commercially in the making of smallbushings. The briquettes are formed on pressure machines of the pilltype.

After forming, the briquettes are packed in oil coke in cast iron boxes,with covers, and of rectangular shape, approximately 6 by 12 inches insize. They are then placed in a cold furnace, electrically or gasheated, and are brought to a temperature of 1425 and maintained at thattemperature for from five to six hours, when they are allowed to cool inthe boxes and are then removed and scratch brushed prior to machining.

One objection that has been encountered is that the product can not besuccessfully machined, owing to the large amount of copper oxide whichoccurs in the bushings, due to the method of sintering. Copper oxidebeing extremely errosive, rapidly Wears and dulls the tools, so that itis impossible to machine the product of the prior process. The presenceof copper oxide also tends to destroy the mechanical strength of thebushing, and it is very liable to break on the mandrel while machiningis attempted.

The method of sizing the bearings in prior use is that of plainbroaching or sizing under pressure with smooth tools. This has theobjectional effect of closing the pores of the bearing to a largedegree, and reduces capacity as a carrier or holder of lubricant Whichis supplied to the bearing.

I have found that the reduction and pre-' from 30 seconds to 2 minutes,at the predetermined temperature," which temperature is governed by themetallic constituents of the composition to be sintered.

By so sintering a bushing the time that it is exposed to hightemperature is decreased and by the use of the reducing gas, theformation of copper or other oxides is prevented. Also any small amountof oxide that may be present in the powder prior to briquetting isreduced.

The prior methods of briquetting call for a substantial amountofgraphite in the mixture. This the present manufacturers perforce mustuse to prevent extreme die wear in forming the briquettes. The use ofthis graphite destroys the mechanical strehgth of the briquette aftersintering, as graphite, being lamellar in structure, prevents theperfect coalescing of the metallic particles during sintering, graphitebeing light and 6% by weight being about 25% by volume.

In my process, however, it is not essential to employ graphite. I use asolid lubricant in molding the briquettes. Such a lubricant is alsovolatile at temperatures very much below that of sintering. This solidlubricant is introduced into the metal powders in a dissolved state. Forexample, of 1% to 5% stearic acid is dissolved in one of its manysolvents, such as ether. The dissolved stearic acid is added to themetallic powder,.and

thoroughly mixed. After mixing, the ether being extremely volatile, iseasily driven ofi, leavlng each finely divided metallic particleentirely coated with a lubricating film. The mix may then be briquettedunder high pressures. The pressure may vary considerably but willusuallybe between 5,000 and 55,000 pounds per square inch. Under. thebriquetting pressure, the finely divided metallic particles beingcovered with the solid volatilizable lubricant, will slidereadily overeach other in arranging themselves relative to each other in theformation of the briquette. The coating of lubricant on the particlesalso serves to prevent oxidationfnBy reason of the lubricant coveringthe metal particles, the briquette will not in anyway abradeor weartools or dies used in the operation. At the same time the push-outpressures may be reduced to approximately one-tenth of the formationpressure. For instance, in the use of graphite as a lubricant in theprior practice, bearings are formed at a pressure of 55,000 ounds, andit takes 40,000 pounds to eject the formed briquette from the die. Withthe use of my lubricant, a bushing formed at 55,000 pounds maybe ejectedfrom the mold or die with a push-out pressure of 5,500 pounds. This isof commercial importance, as it is necessary in the first case towater-jacket dies, owing to excessive heat due to push-out pressures,but it is not necessary in the second instance.

89% of copper with 10% of tin and 1% of stearic acid dissolved in etheris-a specific example of proportions producing a bushing of greatstrength, ductility and machineability;

Furthermore, by the use of stearic acid,

or other solid volatilizable lubricant, I am able to volatilizecompletely the stearic, leaving no residual particles to interposethemselves between the metallic particles. Consequently, I get perfectcohesion between the individual metallic particles on sintering, therebyincreasing the strength over five times that of the sintered bushingusing graphite as a lubricant.

I have also used hydrocarbon oils for this purpose, but find that onsintering residual carbon is left, which materially reduces the strengthof the finished bushing;

My invention therefore involves the use of a solid lubricant, dissolvedand in a liquid state at the time of mixing with the metallicconstituents, the solvent being volatile, and being allowed to evaporateprior to formation of the briquettes, and the solid volatilizablelubricant being volatilized and driven off after the briquettes areformed at a temperature substantially below that of the sinteringoperation. The particles are then in condition to coalesce and cohereperfectly at sintering temperature, producing a very ductile product,readily machined,

substantially free from oxides, and having 1great strength on radial andcompressive cad.

Among other fatty acids that may be used in this connection, as well asstearic, are palmitic and oleic, all of which have lubricatingproperties.

In connection with the use of stearic as a lubricant,it serves not onlyas a lubricant, but by increasing the amount over and above thatvnecessary as a lubricant, one can control the ultimate porosity of thefinished bushing or otherarticle within certain.

ranges. For instance, by using 5% stearic acid, one would, increase theporosity approximately 20%. In this instance of 1% or 1% of stearic willbe required to lubricate while the 4% additional will serve to increasethe porosity. The lubricating 1% of stearic in volatilizing-will alsoleave voids and cause porosity, but additional stearic will materiallyincrease the porosity.

I propose to accomplish the sintering of the metallic briquette y theapplication of electrical heat obtained, through the resistance of themolded metallic article itself, or by eddy currents induced within themolded article by the use of an induction furnace While the articleenveloped in a reducing atmosphere or an atmosphere of a reducing gas,though I do not desire to limit myself in this regard.

According to this method a high voltage electric current is passedthrough or into the metallic briquette while in a suitably ventedchamber containing an atmosphere of the reducing gas, the heat. beingproduced by the resistance of or by eddy currents set up within thebriquette itself.

Suitable reducing gases which may be used are hydrogen, helium, orordinary coal gas commonly known as illuminating gas.

The current when applied by contact with the briquette is passed throughthe briquette from end to end while the same is disposed in the reducinggaseous atmosphere, similarly to the manner in which a welding currentis passed through the pieces to be welded as they are held in the.clamps of an electric welder.

Induction furnaces are well known in the art and can be purchased on theopen market. In these furnaces, the applied primary voltage in aseparate winding causes a secondary current to be set up in thesecondary winding. The briquette may be placed within the secondarywinding and in the path of the duced current passing creates thesintering quette itself.

The briquette formedof the metallic particles coated with the solidvolatilizable lubricant is protected against oxidation by the saidlubricant coating or covering and induced current. Thus, the inthroughthe briquette heat within the briupon heating the briquette in thereducing gaseous atmosphere, the lubricant is volatilized and anycontained or existent oxides are effectively reduced and furtheroxidation is substantially prevented.

I claim:

1. The hereindescribed method of form-,

ing porousbodies from powdered metal comprising dissolving a solidvolatilizable lubricant in a volatile solvent, mixing the solution' withthe powdered metal, volatilizing the volatile solvent until it isremoved leaving each particle of metal powder coated with a lubricatingand oxidization preventing film of the solid volatile lubricant, formingthe mix into a briquette under pressure, and subjecting the briquette tosintering heat in a reducing medium or atmosphere below sinteringtemperature, and to sinter'and coalesce the metal particles metal tometal with voids distributed throughout the mass. 2. The hereindescribedmethod of forming porous bodiesfrom powdered metal comprisingincorporating stearic acid with the lubricant or other foreign materialand with voids distributed throughout the metallic 3. Thehcreindescribed method of forming porous bodies from powdered metalscomprising dissolving stearic acid in a volatile solvent, mixing thesolution with the powdered metal and allowing'the volatile solvent tovolatilize until it is removed leaving each particle of metal powdercoated with a lubricating and oxidization preventing film of the stearicacid, forming the mix under high pres sure into a briquette, and heatingthe briquette in a reducing medium or atmosphere to volatilize and driveofi the lubricant stearic acid at a temperature below that required tosinter the metal, and at sintering temperature to sinter and coalescethe metal particles metal to metal with voids distributed throughout themass.

. 4. The hereindescribed method of forming porous bodies from powderedmetal comprising dissolving a fatty acid in a volatile solvent mixingthe solution with the powdered metal and allowing the volatile solventto evaporate, leaving the particles of metal powder coated with alubricating and oxidization preventing film of the fatty acid, formingthe mix into a briquette under pressure, heating the briquette in areducing medium or atmosphere to volatilize and drive off the fatty acidat a temperature below that required to sinter the metal particles, andon reaching sintering temperature to sinter and coalesce the metalparticles metal to metal with voids distributed throughout the mass.

5. The method of claim 3 in which is used an amount of fatty acid inexcess of 1% of the mix, the 1% of fatty acid being suflicient tolubricate and produce porosity on volatilizing, and the excess beingused to materially increase the porosity of the sintered article, theporosity being controlled by the amount of the excess.

6. The hereindescribed method of forming porous bodies from powderedmetals comprising dissolved stearic acid in a volatile solvent, mixingthe solution with. the powdered metal and allowing the volatile solventtovolatilize until it is removed leaving each particle of metal powdercoated with a lubricating and oxidation preventing film of the stearicacid, forming the mix under high pressure into a briquette, and heatingthe briquette to a sintering temperature.

7. The hereindescribed method of forming porous bodies from powderedmetal comprising dissolving a fatty acid in a .volatile solvent, mixingthe solution with the powdered metal and. allowing the volatile solventto evaporate, leaving the particles of metal powder coated with alubricating and oxidation preventing film of the fatty acid, forming themix into a briquette under pressure, and

heating the briquette to a sintering temperature.

8. The method of forming an article of manufacture which consists inmixing powdered metals together, thoroughly distributin g s'tearic acidthroughout said mixture, compressing the mixture to a briquette form,and sintering the briquette to 'form a hard porous structure and tosimultaneously volatilize the stearic acid.

In testimony whereof, I aflix my signature.

CHARLES F. SHERWOOD;

